1. Field of the invention
The present invention relates to a binder which comprises pullulan and saccharide(s) as a main ingredient, and its uses; more particularly, to a binder having a relatively-high binding capacity and a product formed by incorporating said binder in a material.
2. Description of the Prior Art
Pullulan, a viscous glucan which is obtained by culturing under an aerobic condition a microorganism of the species Aureobasidium pullulans in a nutritional culture medium containing saccharides such as mono- and oligo-saccharides, has been prepared in an industrial scale.
Pullulan has properties such as satisfiable water-solubility, edibility, film-forming ability and binding capacity, and because of these it has been used as a base, adhesive and coating agent in food products, cosmetics and pharmaceuticals, as well as in a variety of fields of formed products or moldings such as a granule, tablet, rod, film and sheet.
In order to improve the binding capacity of pullulan, it has been proposed to remove concomitant saccharides as much as possible, and this proposal has been actually employed. For example, Japanese Patent Laid-Open Nos.105,887/75 and 116,692/75 disclosed a preparation of a high-purity pullulan by adding an organic solvent to a culture.
The products thus obtained, however, had a relatively-high content of pullulan, it could not be readily used in a variety of industrial fields because the products exhibited a relatively-high viscosity and required a relatively-high production-cost and a skillful handling when dissolved in a solution. Also proposed was a binder containing pullulan which can be readily used in an industrial field. For example, Japanese Patent Laid-Open No.172,566/84 proposed a sizing agent which is obtained by culturing a pullulan-forming microorganism in a culture medium containing torrefaction dextrin as a carbon source.
It was found that the content of pullulan in the composition as disclosed in the Example of the above-mentioned publication was less than 10 w/w % (throughout the specification the symbol "w/w %" is abbreviated as "%", if specified otherwise), based on the weight of the dry solid (d.s.b.), and that the binding capacity of the composition was unfavorably low.
The publication pointed out the drawbacks of torrefaction dextrin, i.e. "torrefaction dextrin was poor in bacterial resistance, and moisture proof and frangibility of a coating film formed therewith, as well as in curling resistance of a paper coated therewith."
According to the Japanese Industrial Standards (JIS), torrefaction dextrin should fulfill the following requirements: It exhibits purplish red on the iodine reaction and contains 6% or lower of glucose as a reducing sugar. The amount of glucose corresponds to about 17 as an average polymerization degree of saccharides.
Japanese Patent Laid-Open No. 246,239/86 proposed a viscous composition wherein a viscosity-imparting substance and a plasticity-imparting substance are mainly incorporated in pullulan, and, if necessary there is further added to the mixture one or more agents such as a filler, coagulation-improving agent and viscosity-controlling agent.
It was found that the composition, however, was insufficient in its binding capacity because it was proposed to incorporate therein 50% or higher saccharides, d.s.b., as a filler, and less than 30% pullulan, d.s.b. Therefore, the invention of the publication has not attained the object.
It has been a great demand to establish a binder which contains pullulan, as well as having a relatively-low viscosity, ready handleability and relatively-high binding capacity without a fear of causing an environmental pollution.